The present invention is related to flow sensors and more particularly to packaging for flow sensors.
Flow sensors were used in a wide variety of applications to measure the flow of a gas through a predefined space. Flow sensors such as the Honeywell AWM1000, AWM2000, AWM3000 and AWM5000 Flow Sensors were used particularly in heating, ventilating and air conditioning (HVAC) applications where air flows through a duct were to be measured. The sensor produced an electrical signal representative of the air flow passing by the sensor. Such a signal was then used by a controller (not shown) to determine how best to control air flow through the duct in order to maintain air flow in a controlled space at a desired temperature.
Such flow sensors were located within a package not directly in the flow of the air. The flow to be sensed was directed past the sensor through the use of hoses or tubes. As shown in FIG. 1, one prior art package 10 included inlet 15A outlet 15B restrictor 20 hose 25 and wire lead 30. A sensor 35 was located within the package in an flow channel (not shown) which existed between the inlet 15A and the outlet 15B. Note that the flow included an axis of flow which was co-linear with the axis of flow for the inlet and the outlet. The sensor was electrically connected to wire lead 30 to provide an electrical output in proportion to the flow across the sensor. The restrictor was typically placed in series with the sensor to scale the flow across the sensing device. A less well known purpose was that this restrictor created a low pass filter effect by means of the restriction in combination with the passage way connecting inlet 15A with outlet 15B. The package shown in FIG. 1, will experience errors when subjected to fluctuations in the pressure at the outlet tube 25, in that the filtering effect of restrictor 20 was minimized.
To avoid this problem, the package of FIG. 2 was designed. Note that FIG. 2 depicts a package that was essentially the same as FIG. 1 except that there were no integral restrictors 20 in the package of FIG. 2. Also, external restrictors 40A and 40B and inlet hose 25A are included in this design. This design provides a workable solution around the problem of transitory pressures at the outlet tube. The second restrictor also adds an additional effect to the fluid dynamic properties of a sensing chamber for fluctuations at the low pressure side (the outlet side) of the center. Significant signal to noise improvements have been noted using the dual restrictor design. However, the external restrictors, tubing and labor associated with assembly added substantial additional cost.
Accordingly, it is an object of the present invention to provide an flow sensing package having the benefits associated with the design of FIG. 2 but at a substantially reduced price. It is a further object of the present invention to provide a flow sensor package which reduces the likelihood of damage occurring to the sensor.